1. Field of the Invention
This invention relates to switching power supplies and in particular to an improvement in a control circuit used to control the output voltage of a switching power supply.
2. Description of the Prior Art
FIG. 1 is a schematic diagram of a symmetrical half-bridge switching power supply. It is to be noted that the novel features disclosed herein are directly applicable to other symmetrical power supplies, wherein a switching means drives current symmetrically positive and negative through a magnetic device, and to nonsymmetrical switching power supplies, wherein current is driven through a magnetic device in one direction only.
The power supply circuit of FIG. 1 uses AC-DC rectifier 10 to rectify an AC input voltage. Capacitor C4, in parallel with rectifier 10, filters the rectified AC voltage to provide a DC input voltage V.sub.in at the drain of switching transistor Q1. Switching transistor Q2 is in series with transistor Q1 and has its source coupled to ground. Transistors Q1 and Q2 are alternately driven by regulator circuit 20. Capacitor C5 has a first terminal coupled to the source of transistor Q1 and the drain of transistor Q2 and has a second terminal coupled to inductor L3, wherein inductor L3 is connected in series with primary winding W1 of power transformer T3. When transistor Q1 is switched on and transistor Q2 is switched off, voltage V.sub.in is applied across the series combination of capacitor C5, inductor L3, and winding W1. As indicated by the dot notation on windings W1, W2, and W3 of power transformer T3, when transistor Q1 is on and transistor Q2 is off, a voltage appears across secondary winding W2 of power transformer T3 which forward biases output rectifying diode D2 at approximately 0.7 volts above the voltage across filtering capacitor C7. This rectified and filtered signal is further filtered by inductor L4 and capacitor C8 in order to supply a DC output voltage V.sub.out.
When transistor Q1 is switched off and transistor Q2 is switched on, shorting the first terminal of capacitor C5 to ground, the direction of current through winding W1 reverses and causes the voltage across windings W2 and W3 of power transformer T3 to reverse. The voltage across winding W3 now forward biases output rectifying diode D3 and is filtered by capacitor C7, capacitor C8, and inductor L4 to provide DC output voltage V.sub.out.
Output voltage V.sub.out is sensed by voltage sense/isolation circuit 15, which provides isolation between the input and output sections of the power supply and generates a feedback signal based on the difference between V.sub.out and a reference voltage. This feedback signal is then applied to regulator circuit 20, which adjusts the switching frequency of transistors Q1 and Q2 accordingly to adjust V.sub.out. An increased switching frequency results in a lower V.sub.out mainly due to the increased impedance of inductance L3 and winding W1 at the higher frequencies. The inclusion of capacitor C5 allows a switching resonant frequency for a maximum V.sub.out. The typical regulator circuit used in these types of symmetrical switching power supplies requires a relatively expensive voltage controlled oscillator to generate a switching frequency corresponding to the amplitude of the feedback signal from voltage sense/isolation circuit 15.
In a typical non-symmetrical switching power supply, such as that described in the Background of a copending application entitled, "An Improved Voltage Regulator Circuit", by David T. Carroll, Ser. No. 07/137,787, Filed Dec. 29, 1987, submitted herewith and incorporated by reference, a relatively expensive pulsewidth modulator circuit is required to modulate the duty cycle of a switching means to adjust the power supply output voltage.
The prior art circuit of FIG. 1 also requires a low voltage power suply to power the voltage regulator circuits. This low voltage power is usually solely supplied by tapping a voltage from a voltage divider network coupled between DC input voltage V.sub.in and ground or by a line operated transformer. The low voltage power supply is, in these prior art circuits, inefficient since they generate constant power regardless of frequency, and thus the power not used to charge the gates of the switching power transistors is lost as heat.